The present invention relates to the use of an age-hardenable copper alloy having a selectively adjustable electric conductivity for the manufacture of casting molds, in particular continuous casting molds, wherein molten metal is stirred by the action of electromagnetic forces.
In the continuous casting of steel in particular, it is generally known that an improvement in quality can be achieved by the electromagnetic stirring of the molten mass contained in the cooled continuous casting molds. Using electromagnetic rabbling mechanisms, a desired flow is forced upon the liquid core of the molten metal within the solidified casting shell which prevents segregations from adversely affecting the cast structure of the billet during the solidification process.
During casting, the liquid molten metal is brought, within the rabbling mechanism, under the influence of an electrical rotating field transversely to the billet pull-off direction and set into vertical motion by the resulting induced currents, the motion running essentially concentrically to the longitudinal axis of the billet. As a result, a homogeneous cast structure is obtained which meets especially high quality demands. To keep the technical expenditure as low as possible, rabbling mechanisms are usually arranged underneath the mold so that the remaining molten metal in the partially solidified billet can be stirred immediately under the mold. To also be able to influence the solidifying structure where the outer edge areas of the billet solidify first, it is beneficial to place the rabbling mechanism either at the level of the mold or in the mold itself.
As a rule, the mold materials used in the continuous casting of steel have high thermal conductivity accompanied at the same time by high mechanical resistance in order to assure optimum heat dissipation and cooling capacity. This leads to a high maximum casting speed and increases the economic efficiency of the continuous steel casting. However, in the arrangement of an induction-rabbling mechanism, the high electric conductivity of the proven mold materials, as, for example, copper-chromium-zirconium alloys having IACS greater than 85%, proves to be disadvantageous. The high electric conductivity leads to an undesired high screening effect of the mold material with respect to the magnetic field produced for the purpose of stirring. This weakening of the magnetic field results in a stirring effect which is not as deep-acting. To compensate for this, the stirring action can be strengthened by increasing the current intensity. However, the technical expenditure necessary for that purpose rises disproportionally. Overall therefore, an optimum stirring action with current mold materials having high thermal conductivity is not attainable.
Mold materials having lower thermal conductivity are also already known. However, these mold materials have extremely high thermal resistances so that preferably they are used at higher temperatures. In addition, because of the extremely high thermal resistance, the machining of these mold materials is relatively costly. In addition, a further disadvantage is that the elongation-at-break at temperatures above 350xc2x0 C. is too low.
Consequently the known mold materials having lower thermal conductivity do not represent an economic alternative to the highly conductive mold materials, as, for example, copper-chromium-zirconium alloys, for use in casting installations having an electromagnetic rabbling mechanism.
An object of the present invention is to provide an age-hardenable copper material, in particular for use in casting installations having an electromagnetic rabbling mechanism, the copper material producing a low field damping and furthermore possessing favorable resistance and elongation-at-break properties.
The means for attaining this objective consists in the use of an age-hardenable copper alloy of 0.1 to 2.0% nickel, 0.3 to 1.3% chromium, 0.1 to 0.5% zirconium, up to 0.2% of at least one element from the group consisting of phosphorous, lithium, calcium, magnesium, silicon and boron, the remainder copper and impurities. This invention provides for a selectively adjustable electric conductivity for manufacturing casting molds, in particular continuous casting molds, in cases where molten metal is stirred by the action of electromagnetic forces.